Phytotoxicity factors, herbicide contamination, compost quality

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Renewable Agriculture and Food Systems: 20(2); 67–72 The factors influencing expression of phytotoxicity in composts and their relationship to the topic of herbicide contamination are examined in this paper concerning quality management practices. Herbicide contamination began as an issue in Washington State late 2001, when traces of an auxinic herbicide picloram were found in WSU composts after off-label use on pasture where hay was taken and incldued in animal bedding and composting. The issue mushroomed into clopyralid being implicated – a related auxinic but one with significantly lesser threshold activity than picloram. In this paper, the authors – agronomists and ag chemists- examine briefly the history, the impact on organic production, and create a table based on literature documenting compost use rates, compost maturity issues and herbicide bioassays thresholds. From this they suggest that greater attention to the use of appropriately degraded composts designated as to completeness or maturation, and applied at appropriate use rates, would minimize the potential for phytotoxic effects, irrespective of the contributing source.